chemical sensing

1
chemical sensing

• linear devices

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chemical sensing

• introduction to chemical sensing and sensors
• vapor detection techniques (mostly chemistry)
• bulk detection techniques (mostly physics)
• in general, spectroscopic techniques
• in parallel, algorithmic approaches to
  signal-to-symbol transformation forthese sorts
  of signals

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approaches to chemical sensing

• identify the nuclei, e.g.,neutron-activation or
  ?-ray spectroscopy
• identify the atoms, e.g.,flame emission
  spectroscopy
• these are great for, e.g., prospecting for iron
  ore you might not care whether you find FeO,
  Fe2O3, Fe2S3, or any other iron compound, as long
  as it is Fe
• but if you want to know, e.g., how a computer
  works it doesnt do you a lot of good to grind it
  up and analyze the dust for H, C, N, O, Si, Al,
  Fe, Cu, Au, Sn, Pb, etc

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• identify positive and negative ions of atoms,
  fragments of molecules, most small molecules,
  some big molecules, e.g., mass spectrometry
• but there are many ways to make the same mass,
  e.g., H3COCH3 (acetone) and H3CCH2OH (ethyl
  alcohol) look the same at any practical mass
  resolution, and both look the same as NO2 and
  isotopes of Ca, Sc, Ti, and V (all atomic mass
  46) at low resolution, i.e., at high detection
  sensitivity

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• identify effect of molecular solubility
  (partition) between two solvents on transport
  time through a sticky pipe, e.g., gas and
  liquid chromatography
• retention time not unique
• concatenated techniques, e.g.,GC-MS, effective
  but slow and expensive
• identify electric-field induced drift rate of
  molecular ions through a gas, e.g., ion mobility
  spectrometry (IMS, plasma chromatography, )
• airport hand luggage sniffershttp//www.sensir.c
  om/Smiths/InLabSystems/IonScan/IonScan.htm

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• identify characteristic x-ray spectral
  attenuation of materials of particular interest
  in particular places
• airport color x-ray machines for explosives,
  drugs
• and probably a hundred specialized technologies
  relying on ...
• photoelectric effect
• speed of sound
• infrared absorption
• etc etc etc ... taking advantage of some unusual
  chemical or physical property of the specific
  analyte

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• in general, we can do quite well these days with
  complex instruments whose scale is room size or
  even desk size ... and more recently, desktop
  monitor size ...
• but there is a demand for low-cost hand-held (or
  robot-held) equivalents
• many are based on chemi-resistors,
  chemi-transistors, chemi-capacitors, etc
• covered briefly on the white-board recently
• first we will discuss laboratory chemical
  analytical instruments and how they are
  being/might be miniaturized

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spectroscopies
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spectroscopies

• when a single component produces a mix of
  separable responses ...
• example the optical spectrum of a particular
  isotope of iron (Fe)
• electron state transitions between all possible
  energy levels of the atom (subject to some
  selection rules)
• example the ion mass spectrum of a molecule of
  heptane (gasoline is mostly C7H16)
• C, CH, CH2, CH3, CH3C, CH3CH, CH3CH2,
  CH3CH2C, CH3CH2CH, CH3CH2CH2, CH3CH2CH2C,
  ..., CH3CH2CH2CH2CH2CH2CH3

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• or a mixture produces a complex response for
  each component
• can sometimes pre-separate the mixture
  components
• gasoline ..., hexane (C6H14), heptane (C7H16),
  octane (C8H18), ... can be separated in time
  domain (e.g., gas chromatography)

(structural separation by MS)
(temporal separation by GC)
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optical spectroscopy
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illustrates the general principle

• inevitable tradeoff between your ability to
  separate spectral components (resolution,
  selectivity) and your ability to detect small
  quantities (sensitivity)

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miniaturization example
Ocean Opticsoptical spectrometeroptics and
electronicson a PC card separatelight source
(below),and fiber optic (blue)light input path
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example VIS-NIR Diffuse Reflectance Spectrum to
Measure Fish Freshness
(probe light in and out)
(monochromator specific color light out)
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mass spectrometry
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mass spectrometry

• usually a separation based on mass of positive
  ions sometimes negative ions, rarely neutrals
• usually all the ions are accelerated to the same
  energy (and filtered to remove outliers)
• velocity thus depends on mass v (2 W/m)1/2
• velocity measured by time-of-flight, by
  trajectory in a magnetic field, etc, in many
  different geometries

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• smaller lower cost alternativequadrupole mass
  spectrometers
• ions move under combined influence of DC and
  oscillating (RF) electric fields most orbits are
  unbounded, but for any particular mass there is a
  small region in the DC/RF amplitude plane where
  they are bounded
• equations of motion analogous to the inverted
  pendulum
• similar to the inverted pendulum application
  made famous as an example of fuzzy logic control

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miniaturization example

• argon/air/helium, 500 micron diameter rods, 3
  cm longhttp//www3.imperial.ac.uk/portal/page?_pa
  geid189,618267_dadportallive_schemaPORTALLIVE

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chromatographies
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gas chromatography

• pipe coated (or packed with grains that are
  coated) with a sticky liquid (stationary
  phase)
• inert gas (e.g., He) flows through the pipe
  (column)
• mixture (e.g., gasoline) squirted into head
• gas (mobile phase) carries it over the liquid
• mixture components move at different effective
  speeds due to different equilibria between phases
• components emerge at column tail
• detect with a universal detector
• or use as inlet to mass or optical spectrometer,
  etc

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miniaturization example

• http//eetd.llnl.gov/mtc/Instruments.html(another
  instrument fewer details link to this one
  has disappeared)

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MANY similar techniques

• liquid chromatography
• liquid mobile phase, solid or liquid stationary
  phase
• ion mobility chromatography
• ion drift velocity through a gas under influence
  of an electric field (airport explosives detector
  principle)
• electrophoresis
• molecules drift through a gel under influence of
  an electric field (used in many medical tests)
• real old fashioned chromatography
• dye-like chemicals separated by different
  diffusion speed through a packed powder, e.g.,
  chalk stick,or soup dribble on table cloth

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hybrid techniques
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hybrid or tandem techniques

• for routinely detecting and identifying any but
  the simplest chemical species, hybrid techniques
  are usually employed
• GC MS
• pre-concentration IMS (airport explosives)
• multiple MS stages with collisional
  decomposition between stages
• etc

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LC MS with high-pressure ionizer etc
note analogy to image processingnot one magic
bullet, but a cleverchain of simple unit
operations
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linearity
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linearity superposition

• all the techniques discussed today are (nearly)
  linear in several senses of the word
• output signal linear in sample concentration
• response to multiple components present
  simultaneously is the sum of the responses to the
  individual components separately
• i.e., little or no cross-sensitivity
• later we will discuss sensors where this is not
  true, e.g., solids state chemical sensors
• like the SnO2 chemi-resistors discussed
  previously
• if it is true then simple pattern recognition
  works

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unraveling overlapping spectra(or signatures)
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overlapping spectra of a mixture

• absent separation (like GC), given the spectrum
  of a mixture, how best to unravel its components
  when the component spectra all overlap?
• arrange your spectrum library in a rectangular
  matrix
• S1 s11, s12, s13, ..., s1n1 hexane,
  1,2,3,...,n peak IDs
• S2 s21, s22, s23, ..., s2n2 octane,
  1,2,3,...,n same peak IDs
• ... etc ....
• Sm sm1, sm2, sm3, ..., smnm Xane,
  1,2,3,...,n same peak IDs

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• consider the inverse problem given the
  concentrations, it is very easy to predict what
  the combined spectrum will be
• C c1, c2, c3, ..., cm,1 hexane, 2
  octane, ..., m Xane
• S c1S1 c2S2 c3S3 ... cmSm
• or in matrix notations c S

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• if we look at exactly as many spectral peaks as
  there are components in the mixture then the
  matrix is square, and it is easy c s-1 S
• if we have fewer peaks than components then we
  are up the creek
• well, we can establish some constraints ...
• if we have more spectral peaks than components
  in the mixture then what to do?
• more peaks than components means we haveextra
  data that we can use to improve theprecision of
  our result a sensor fusion opportunity

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pseudo-inverse method

• the trick is to multiply both sides of the
  equation by sT
• s c
  S(npeaks ncomponents) (ncomponents 1)
  (npeaks 1)
• sT s
  c sTS
  (ncomponents npeaks) (npeaks ncomponents)
  (ncomponents 1) (ncomponents npeaks)
  (npeaks 1)
• note that sTs is square, so it (generally) has
  an inverse

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• c (sTs)-1-sTS (ncomponents 1)
  (ncomponents ncomponents)(ncomponents
  npeaks) (npeaks 1)
• the calculated component concentrations are
  optimal exactly the same as least squares
  fitting
• i.e., algebraic least squares fit gives the same
  resultas matrix solution using pseudo-inverse
  formalism
• yes, of course, there are degenerate cases where
  sTs doesnt actually have an inverse, or
  calculating it is unstable
• then you need to use better judgement in
  deciding which peaks to use!

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caution ...

• c (sTs)-1sTS is the same as the optimal result
  you would get if you minimized the sum of the
  squares of the differences between the components
  of the data set S and a predicted data set S
  s c
• ?? Sum((sc - S)i over all npeaks spectral
  peaks)d? /dcj 0 gives ncomponents simultaneous
  equations which when you solve them for c gives
  the same result as the pseudo-inverse

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• but (to keep the notation and discussion simple)
  Ive left out something importantas in our
  previous discussion about how to combine multiple
  measurements that have different associated
  uncertainties, you need to weight each datum by a
  reciprocal measure of its uncertainty, e.g.,
  1/?i2(in both the least-squares and the
  pseudo-inverse formulations)
• specific ad hoc weighting schemes are often hard
  to justify with first-principles arguments

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exercise

• the following table shows the major peaks in the
  mass spectrum of a mixture of FC-43 and FC-70
  you can find their individual spectra at
  http//www.sisweb.com/index/referenc/mscalibr.htm
  use the EI Positive Ion ... data estimate the
  fractions of FC-43 and FC-70 in this
  mixturefirst do a quick and dirty estimate,
  then do it as precisely as you can given the data
  at your disposal do you get the best result by
  using all the data, or might it be better to
  discard, e.g., data from some of the smaller
  peaks?

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note amu meansatomic mass units (called
daltons, by chemists and biologists) all the
peaks are normalized to the biggest one(CF3 ?
69 amu)